The present invention relates generally to inkjet apparatus, including inkjet printing mechanisms, and more particularly to an improved mechanism for avoiding print head crashes in such apparatus.
Inkjet printing mechanisms may be used in a variety of different inkjet apparatus, such as plotters, facsimile machines, copiers, and inkjet printers collectively referred to in the following as printers, to print images using a colorant, referred to generally herein as xe2x80x9cinkxe2x80x9d. These inkjet printing mechanisms use inkjet cartridges, often called xe2x80x9cpensxe2x80x9d or xe2x80x9cprint headsxe2x80x9d to shoot drops of ink onto print media, which can be used in the form of cut sheets or rolls of print media, which may include paper, vinyl, films, canvas or the like, in a variety of different dimensions.
Some inkjet print mechanisms carry an ink cartridge with an entire supply of ink back and forth across the sheet. Other inkjet print mechanisms, known as xe2x80x9coff-axisxe2x80x9d systems, propel only a small ink supply with the print head carriage across the print zone, and store the main ink supply in a stationary reservoir, which is located xe2x80x9coff-axisxe2x80x9d from the path of print head travel. Typically, a flexible conduit or tubing is used to convey the ink from the off-axis main reservoir to the print head cartridge. In multi-color cartridges, several print heads and reservoirs are combined into a single unit, with each reservoir/print head combination for a given color also being referred to herein as a xe2x80x9cpenxe2x80x9d.
Each pen has a nozzle plate that includes very small nozzles through which the ink drops are fired. The particular ink ejection mechanism within the print head may take on a variety of different forms known to those skilled in the art, such as those using piezo-electric or thermal print head technology. For instance, two earlier thermal ink ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481, both assigned to the present assignee, Hewlett-Packard Company. In a thermal system, a barrier layer containing ink channels and vaporization chambers is located between a nozzle orifice plate and a substrate layer. This substrate layer typically contains linear arrays of heater elements, such as resistors, which are energized to heat ink within the vaporization chambers. Upon heating, an ink droplet is ejected from a nozzle associated with the energized resistor.
By selectively energizing the resistors as the print head moves across the sheet, the ink is expelled in a pattern on the print media to form a desired image (e.g., picture, chart or text). The nozzles are typically arranged in one or more linear arrays. If more than one, the two linear arrays are located generally side-by-side on the print head, parallel to one another, and substantially perpendicular to the scanning direction. Thus, the length of the nozzle arrays defines a print swath or band. That is, if all the nozzles of one array were continually fired as the print head made one complete traverse through the print zone, a band or swath of ink would appear on the sheet. The height of this band is known as the xe2x80x9cswath heightxe2x80x9d of the pen, the maximum pattern of ink that can be laid down in a single pass.
For placing further print swaths on the print media, a print media feed mechanism is employed to advance or index the medium in the print zone in a second direction, called the media direction, which is usually substantially perpendicular to scanning direction of the print head.
Thus, to print an image, the print head is scanned back and forth across a print zone at a very close distance above the sheet, with the pen shooting drops of ink as it moves. On one hand, for instance, the distance between the printhead and the paper must be as small as possible, for example less than 1.7 mm, in order to obtain an accurate positioning of the ink dots projected from the printhead and to avoid spraying artefacts.
However, when a lot of ink is placed on some print media (especially on low cost paper based media) the print media may be subject to a phenomenon known as xe2x80x9ccocklexe2x80x9d. In existing printers, cockle results from the print media swelling and expanding as it absorbs water contained in the ink, whilst the print media is simultaneously constrained against lateral expansion due to being gripped at given locations along the scan axis (i.e. along the axis of movement of the print head), between the pinch wheels and the main drive roller. Thus, the effect of wet cockle increases with the amount of ink deposited on the paper.
This results in the formation of undulations or wrinkles in the plane of the print media. As a consequence, the distance between the print media and the print head decreases at some localized points. This phenomenon is especially noticeable when printing area fills of more than 200%. By this it is meant that in a given area of print media, the amount of ink deposited during the printing operation is two or more times the quantity of ink that is required to cover that area. This problem is further exacerbated by high temperatures and high levels of humidity.
If the degree of cockle is particularly severe, a xe2x80x9cbubblexe2x80x9d in the media may form. If the height of the media bubble is sufficient, the plot may be damaged as ink on the plot is smeared by the print head. Indeed, in more severe cases, a media crash may occur as the print head impacts against the print media itself. A media crash may seriously affect the subsequent print quality or throughput of the printer due to damaging the operation of individual nozzles of the pen. In some cases a media crash may necessitate the replacement of the pen.
This problem is often of particular concern where a plot is printed on a single sheet of print media, where the problem may be particularly pronounced in the trailing edge, of the sheet of print media; i.e. the last area to be printed. This situation is illustrated schematically in FIG. 1. FIG. 1a illustrates a sheet of print media 10 during a print operation. The print media 10 is resting on a platen 400 as it is driven through a printer drive mechanism 30, consisting of a drive roller and opposing pinch wheels, in the direction indicated by the arrow. As can be seen from the figure, the media has expanded laterally, as indicated by arrows xe2x80x9cAxe2x80x9d after having passed through the print zone 40 and as a result of having absorbed the moisture in the ink deposited on it. However, where the media 10 is gripped between the drive roller and opposing pinch wheels of the printer drive mechanism 30 it is constrained against such lateral expansion. However, as can be seen from the figure, the edges of the print media 10 which have yet to pass through the printer drive mechanism 30 have a tendency to align themselves at the same angle as the edges of the print media 10 at the print zone side of the printer drive mechanism 30. In the figure, this is indicated by arrows xe2x80x9cBxe2x80x9d. This has the effect of causing the edges of the print media 10 which have yet to pass through the printer drive mechanism to move towards each other, thus causing the raised zones, which together resemble a xe2x80x9cwavexe2x80x9d in the print media 10. The wave is indicated by arrow xe2x80x9cCxe2x80x9d. The form of the wave is more clearly shown in FIG. 1b which is a cross sectional view of the print media shown in FIG. 1a, taken along lines Xxe2x80x94X.
FIG. 2 illustrates a section of the printer drive mechanism 30 of FIG. 1, illustrating the interrelationship between the platen 400, the drive roller 300 and a series of pinch wheels 310 of the drive mechanism 30. As the print media (not shown) passes the drive roller 300 and the series of pinch wheels 310, it may retain the wave shape xe2x80x9cCxe2x80x9d that it had acquired, as shown in FIG. 1, where it is not constrained between the pinch wheels 310 and the main drive roller 300; i.e. at the locations 70. This results in the formation of media bubbles in these areas. With increased quantities of ink deposited on the sheet of print media 10, such media bubbles expand. This causes the height of the media bubbles to increase and so increasing the likelihood that the ink on the plot may be smeared by the print head, or that a media crash will occur, as has been described above.
The size and number of media bubbles may be reduced by increasing the proportion of the width of the print media (along the scan axis), which is constrained between the main drive roller and the pinch wheels. However, it has been observed that by doing so print media handling problems arise as a result of increased stresses building up in the print media.
This problem may be partially or wholly overcome by using print media that is not susceptible to cockle, or by constraining the print media in the lateral sense in the area before entry into the print drive mechanism. For example, by using a roll fed print media with back tensioning force, the media tends to keep flatter on the platen. Thus, the development of the wave xe2x80x9cCxe2x80x9d is inhibited to a certain extent. However, depending upon the operating conditions, this problem still occurs with such an arrangement. Alternatively, the print media may be entrained around the main drive roller; i.e. using a high xe2x80x9cwrap anglexe2x80x9d. This inhibits the development of the wave xe2x80x9cCxe2x80x9d more effectively. However, such solutions are clearly not appropriate to all designs of printer or modes of operating them. For example, the use of a wrap angle prevents the printer from being used with non-flexible print media.
Another known approach to addressing this problem uses xe2x80x9cskisxe2x80x9d or xe2x80x9cguidesxe2x80x9d located between the pinch wheels of the drive mechanism that force any media bubbles between adjacent pinch wheels to flatten as the print media passes between the pinch wheels and the main drive roller. These xe2x80x9cskisxe2x80x9d or xe2x80x9cguidesxe2x80x9d consist of planar guide surface located between adjacent pinch wheels. Each ski is angled to progressively flatten a media bubble as it advances towards the pinch wheels. In this way, the available space for a media bubble to exist in decreases as the media bubble approaches the pinch wheels, until the point where the media bubble is limited to a height less than that which is likely to cause a media crash.
However, this solution suffers from several disadvantages. Firstly, the print media is sometimes damaged where it comes in to contact with the skis. This problem is particularly noticeable when the print media is glossy or has another surface which is susceptible to surface damage or where such damage is readily noticed. Secondly, in order to effectively limit the height of any media bubbles that have formed, known skis tend to substantially fill the area between adjacent pinch wheels. This has the effect of obstructing the leading edge of a new sheet of print media from the point of view of the user when it is being introduced between the pinch wheels and the main drive roller prior to printing. Furthermore, any sheet alignment marks or lines provided on the printer platen to help the user to correctly introduce a new sheet may also be obstructed from the view of the user by the skis. Thus, the provision of such skis in a printer may make it difficult for the user to ensure that the new sheet of print media is introduced correctly.
In view of the foregoing, it is an object of the present invention to provide an improved inkjet apparatus.
A further object of the invention is to provide an inkjet apparatus for reducing the likelihood of a print head crashes, particularly when printing on pre-cut sheets of print media.
Still another object of the present invention is to provide an inkjet apparatus for reducing the damage to prints in which media bubbles have developed as it is handled by the drive mechanism of a printer.
To achieve these objects, there is provided an inkjet apparatus in which there is a reduced likelihood of a media crash occurring when media bubbles have developed. According to the present invention there is provided an ink jet apparatus having a print media feed path and a print media feed assembly, said feed assembly being arranged to feed print media having a width and a length in the direction of said media length along said feed path, said feed assembly including: a print media drive roller having a rotational axis extending substantially transverse to said feed path; first and second pinch wheels to rotatably cooperate with said drive roller so as to grip said media therebetween, and being further arranged to grip said media at a first and second respective locations spaced apart along said media width; and, a roller element being arranged to rotate about a rotational axis substantially parallel to said drive roller rotational axis, the roller element being located at a third location along said media width, said third location being substantially between said first and said second locations, the roller element being arranged to limit the height of said media in the region of the third location.
The inter-pinch wheel arrangement of the present invention provides a means of allowing the print media to expand in a controlled manner. Thus, by controlling the height to which a media bubble may grow, the situation where the print head may come into contact with the ink already deposited on the surface of the print media, or the print media itself, may be avoided. Additionally, by allowing the print media to expand in a controlled manner, the stresses induced in print media as a result of the absorption of fluid from ink deposited on it are reduced. Thus, the paper handling difficulties are avoided.
Furthermore since the inter-pinch wheel of the present invention is able to rotate about its axis when it is contacted by a media bubble, negligible relative movement between the surface of the inter-pinch wheel in contact with the print media and the print media itself arises. This aspect of the inter-pinch wheel of the present invention greatly reduces the likelihood of damaging the surface of the print media in which a media bubble has formed. The fact that the inter-pinch wheels of the present invention are free to rotate in the sense of the media advance also allows them to be relatively small in comparison to a ski, whilst being able to adequately control media bubble growth without damaging the media surface. This means that an operator of a printer equipped with inter-pinch wheels of the present invention is able to clearly see any alignment marks on the platen of the printer that facilitate the loading of the new sheets or rolls of print media onto the printer.
Preferably, inter-pinch wheels of the present invention have an undulating profile. This allows a greater degree of media expansion whilst effectively limiting the height of the media bubble than is the case with skis, which due to their comparative difficulty of construction and mode of operation have been used with a flat profile.
Preferably, the inter-pinch wheels of the present invention are manufactured in an injection molding process from a plastics material. Therefore, they benefit from being simple and cost effective to manufacture.
Preferably, the inter-pinch wheel of the present invention is mounted in a xe2x80x9csnap fitxe2x80x9d manner (i.e. pressed into place) on stub axles protruding from adjacent pinch wheels. Therefore, they may be accurately positioned in a simple and cost effective manner.